The present invention relates generally to an improved composition and/or formulation and method for the treatment of metallic surfaces, and more specifically to an improved aqueous formulation adapted to be employed in a working solution for use as a seal rinse for increasing the corrosion protection of painted metallic substrates. The seal rinse formulations of the present invention comprise morpholine and (2-benzothiazolylthio) succinic acid in an effective concentration to form a morpholine salt, the formulations being highly suited for use with surfaces of metallic substrates which have been previously phosphatized with typical phosphate baths.
Techniques for cleaning and preparing metals, particularly ferrous metals prior to coating with an organic top coat have long existed. Effective cleaning is necessary in order to promote adhesion of the organic coating to the metallic substrate and to increase the corrosion protection available for the painted product. Preparation of metallic surfaces involves initially cleaning and/or removing all organic soils, inorganic soils, and particulate material. Organic soils may be present in the form of petroleum oils and greases, polymers, or soaps. Inorganic soils may be present in the form of rust, smut, scale and/or particulate material. Following the cleaning operation, the metallic substrate is exposed to a phosphatizing bath where, in the case of ferrous materials, an iron phosphate surface coating is developed. The primary purpose of the phosphate coating is to convert the normally conductive surface to one which is essentially non-conductive. The non-conductive character minimizes or eliminates what is typically characterized as "flash rusting" which frequently occurs between the time an article has been cleaned and prior to the time that the surface of the article may be painted or otherwise covered with a protective and durable film. The non-conductive character further reduces the rate of corrosion of the surface of the ferrous substrate following painting. For non-ferrous materials, the phosphatizing bath micro-etches the metal substrate and tends to neutralize any alkalinity present on the metal from exposure to previous treatments or otherwise.
The formulations of the present invention are particularly adapted for use as a seal rinse on metallic surfaces following phosphate treatment. Two techniques are typically used for phosphatizing ferrous substrates, these being zinc phosphatizing and iron phosphatizing. Zinc phosphatizing requires careful process control, since the operating parameters are normally quite narrow. In addition, and perhaps more seriously, zinc phosphatizing contains nickel and may also contain manganese, copper, or others in addition to zinc. Control of adverse impacts on the environment provide extensive limitations upon the use of such processes. Iron phosphatizing is generally recognized as far more acceptable, and may in certain instances, be used in combination with accelerating agents such as alkali metal chlorates or bromates, or organic type oxidizing agents. The formulations of the present invention have been found to function exceptionally well when used on surfaces following iron phosphatizing treatment.
In addition to the use of chlorates and bromates, iodates may be employed. Ammonium salts may also be employed in the definition of alkali metals. The presence of the accelerator components promotes a heavier phosphate coating, normally in the 40-70 mg/ft..sup.2 range. Molybdated products also produce some benefits due to cross-plating onto ferrous substrates. However, such products do not impede the rate of corrosion of a ferrous substrate as effectively as phosphate coatings utilizing alkali metal halidates such as chlorates or bromates as accelerators.
Seal rinse formulations for treatment of metallic surfaces have typically involved the following:
1. Chromate's/heavy metals; PA1 2. Phenol formaldehyde resins; PA1 3. Polymeric - chemicals; PA1 4. Phosphoric acid based products; PA1 5. Deionized water/reverse osmosis water; PA1 6. Tannic acid/tannates; or PA1 7. Styrenes.
Each of these have limitations and/or are currently prohibited for use, with some of the disadvantages stemming from the following. While chromates have provided the best overall protection in the past, the effluent discharge levels imposed by the EPA (Environmental Protection Agency) because of their carcinogenic effects and for other reasons, the use has become extremely limited. Heavy metals such as zinc, lead and copper generally provide a lower degree of corrosion protection than chromium, and their disposal also presents limitations upon use. Other metals or salts of metals including molybdates, tungstates, vanadates, zirconium, and the like have provided certain benefits in inhibiting flash rust on ferrous substrates, however their ability to provide corrosion protection is generally limited. When modified to provide corrosion protection of certain painted metallic substrates, their uses tend to be extremely limited, due to the coating-specific nature of their performance.
Certain non-chromate and non-metallic seal rinse formulations which have been developed may suffer from disadvantages due to the current regulations imposed by Governmental agencies. Recently, one final rinse which has been recognized is deionized water, or water prepared from reverse osmosis operations, with both of these having a generally neutral pH and the absence of dissolved solids. These tend to be costly and hence not generally economically feasible, and hence not widely practiced.
Phosphoric acid based seal rinses, as previously discussed, provide resistance to flash rusting, but are generally considered weak for corrosion protection of painted metallic substrates. Other rinses including certain organic acid based rinses have limitations demonstrated in either resistance to flash rusting or, in providing only limited long-term corrosion protection.